1. Field of the Invention
The invention generally relates to photodetection, and more particularly, to photodiode imaging devices.
2. Description of Related Art
Imaging devices such as scanners, photocopiers, or radiation imagers can use a sensor sheet comprising a two-dimensional layer of photodiodes which senses a light image directed onto it. Each photodiode is connected to a transistor. When light strikes a particular photodiode, an electrical charge is generated by the photodiode. The transistor switches the charge generated by the photodiode to other components such as an amplifier which amplifies the signal.
Conventionally, an active matrix array is a grid of pixels on a substrate with an active switching device, such as a thin film transistor. An active matrix imaging array is a grid of pixels on a substrate, where each pixel has a photodiode and a transistor formed on the same substrate surface. The transistor is typically located adjacent to the photodiode. The resolution of the active matrix array is determined by the number of pixels per unit area of the substrate surface. When the resolution of the active matrix array is increased, the size of each pixel decreases leaving less area for the photodiode, the transistor and the lines connecting the photodiode and the transistor.
When the transistor and connecting lines are not proportionally reduced in size, the size of the photodiode must be reduced by a greater proportion so that the transistor, the connecting lines and the photodiode will all fit on the pixel. This reduces the percentage of the surface area of the pixel covered by the photodiode. This percentage is the fill factor of the photodiode imaging device.
As the fill factor decreases, the amount of light received by the photodiode of each pixel also decreases thus reducing the quality of the photodiode imaging device. Further, as the fill factor decreases, the pixel undersamples the image, which accentuates the Modulation Transfer Function (MTF) at high spatial frequencies. This can increase the appearance of image artifacts like aliasing at high spatial frequencies and effectively degrades image resolution.
Moreover, when the widths of the connecting lines are reduced as the surface area of the pixel decreases, the operating speed of the photodiode imaging device is also reduced. This occurs because a reduction in the width of the connecting lines limits the current carrying capacity of the connecting lines and thus reduces the speed at which the electric charge in the photodiode can be transferred through the transistor to other components of the photodiode imaging device.